Which version of Unbuntu is best to install for overall server performance including REAL Studio, WE / Xojo applications?

I realize RS / Xojo apps need 32 bit libraries to run but I don't know if it's better to install the 64bit version of Ubuntu server then install the 32bit libraries or just to use the 32bit version of Ubuntu server and then later need to re-install the 64bit version of the server.

SOME DAY Xojo apps will be 64bit and I'd not want to do a complete OS install just to run Xojo apps in 64bit.

It really doesn't matter. If you install 32-bit, then you don't need to install the libraries; if you install the 64-bit, you do. Otherwise, it's really up to you whether or not you need 32/64-bit for whatever else you're doing with Linux.